1. Field of the Invention
The present invention relates to the art of man-made fiber reinforced materials. In addition, the invention relates to methods of fiber arrangement and composite assembly.
2. Description of the Prior Art
Wood is a naturally occurring composite material formed of hollow tubular cells bound together by lignin, the cell walls being formed by helically oriented cellulose fibers.
Plywood is a partially man-made material in which the natural orientation of wood is varied by specified laminate thicknesses and grain orientation to enhance or develop desired sheet properties.
Over the past twenty-five years, the industrial capacity to form extremely thin, flexible and remarkably strong fibers from glass, carbon and other materials has given rise to new and entirely man-made composites of glass (carbon, etc.) fibers in oriented laminates, woven into a cloth or laid in a randomly oriented mat in which the intersticies between the fibers are saturated with a thermoplastic or thermosetting polymer in the viscous liquid state which is thereafter thermally or chemically cured to the solid state. Rigid structural materials and articles fabricated in this manner have been generically termed "fiber reinforced plastic (FRP) composites."
In the past, engineers and designers have been chiefly concerned with the limits of breaking stress of a material, that is to say, determinations of the force required to produce a given area of fracture surface as measured in pounds per square inch (p.s.i.) or Newtons per square meter (N/m.sup.2).
More recently, it has been found more fruitful to consider not only the force but the energy required to produce a fracture-surface. This energy per unit area term is called the work-to-fracture value of a material and has been found to give a better indication of the performance of a material under actual load conditions than does the breaking stress alone. It is usually the best way of predicting damage tolerance.
"Work-to-fracture" or "work-of-fracture" is analagous to several other expressions of the effective toughness of a material, such as "fracture toughness" or "critical stress intensity."
Various tests are used to determine work-to-fracture. For FRP laminates, the work, which derives mostly from delamination and the mechanism of fiber pull-out, can be measured by pulling a sample apart in a tensile tester. For other, tougher, materials, the work-to-fracture is traditionally assessed by "charpy" or "Izod" tests which involve swinging a heavy pendulum so as to fracture a short beam of the material that has been notched on the tension side. The ratio (a) of work (foot-lbs. or Joules) required to fracture the notched composite short beam in the Charpy/Izod tests to (b) cross sectional area (square inches or square meters) in the exposed crack surface is computed as the material's work-to-fracture.
Typical work-to-fracture values for some common materials include: unreinforced paper at 5.times.10.sup.2 J/m.sup.2 which indicates the material tears readily, wood at 10.sup.4 J/m.sup.2 which is an acceptable work-to-fracture value provided working stresses are moderate, and ductile aluminum and steel at 10.sup.5 -10.sup.6 J/m.sup.2 which is an excellent level of work-to-fracture, indicative of a high degree of damage tolerance.
Although many thermoplastics like nylon have work-to-fracture values which are high enough for use in non-critical applications, their stiffness is unavoidably inadequate for large panels. Conventional solid, FRP composites such as fiberglass, which are stiffer, have a marginally acceptable work-to-fracture value of 10.sup.3 -10.sup.4 J/m.sup.2. This, however, is considered inadequate for many load-bearing applications which are subject to impact. A fundamental problem is that the work-to-fracture of conventional FRP composites is such as renders structures vulnerable to catastrophic failure.
It is important to recognize that while a great deal of effort has been expended in the development of FRP composites which have high stiffness and strength, until recently comparatively little research has been done in an attempt to make such materials sufficiently tough or damage-tolerant to compete with the ductile metals whose superior ability to deform and absorb energy without cracking under stress makes them "safe" for many structural applications.
In developing lighter structures, designers are constantly seeking materials to operate at even high stresses while at the same time there is a need for higher specific stiffness to resist buckling under the action of compressive stresses. In the development of high strength steels it has long been recognized that, in general, high strength can only be achieved at the expense of ductility and as a consequence, toughness. Lower work-to-fracture coupled with higher working stresses means that such materials must be used only in components not subject to heavy impact and where the size of defects can be reliably controlled. Fiber composites exploit the potential high strength of essentially brittle materials. Furthermore, those relatively few materials of high specific stiffness (boron, carbon, silicon carbide, etc.) are by their nature essentially brittle, behaving elastically to the point of failure, and can only be utilized in fibrous form. The reason for this is that in the fiber reinforced composite the naturally occurring defects in individual fibers, though leading to premature failure of that fiber, will not precipitate catastrophic failure of the composite as a whole. This is due to the comparatively low strength of the matrix material and the fiber matrix interface. This mechanism renders fiber composites sufficiently insensitive to microscopic defects for very high strengths to be developed. However, in their conventional forms, the work-to-fracture is inadequate for those applications which require a high degree of damage tolerance.
Bending stiffness may be defined as the product of (1) the Young's modulus of the material (E) and (2) the second moment of area (I) function of the structure geometry. For a given structural member and functional task, it is the engineer's art to coordinate the structural geometry (I) with an appropriate material selection (E) to achieve the stiffness required. Normally, however, material selections are dictated or at least restricted by cost or environmental concerns thereby leaving the engineer with only the geometry of the member as a variable parameter. But when stiffness is achieved by geometric design, weight of the resulting member is directly related. In vehicular applications such as airplanes, automobiles and vessels, weight is of prime concern. For this reason, the vehicle designer must also consider the specific gravity of the material he selects.
Briefly described thus far have been the interrelationships of strength, toughness, stiffness and weight. Although no single test accurately reflects a harmonious blend of these material properties to guide a designer's selection, the calculated property of specific work-to-fracture is extremely helpful. This property of a particular material is obtained as the quotient of the material work-to-fracture property described previously, divided by the specific gravity of the material.
The Table I that follows implements the comparison of the above described material properties respective to several, commonly used, engineering materials.
By comparing the respective properties of mild steel and a hardened steel, it will be noted that although the strength of a given member may be improved by alloying and heat treating, the resulting gain will be at the expense of toughness (see Table I example 1; example 2, condition b; example 3, condition a). If rigidity is gained through geometric design with mild steel, toughness will be retained but at the cost of increased weight and complexity.
Note also from this Table I that both, mild steel and wood, as measured by specific work-to-fracture, are both high toughness materials.
Failure analysis of the strength and toughness properties obtained from prior art FRP composites reveals that frictional energy due to fiber pull-out from a confining matrix accounts for a majority of work-of-fracture absorption required of a structural failure.
TABLE I __________________________________________________________________________ Specific Young's Tensile Failure Work-to- Work-to- Specific Modulus Strength Strain Fracture Fracture Gravity .times. 10.sup.5 N/mm.sup.2 N/mm.sup.2 % .times. 10.sup.4 J/m.sup.2 .times. 10.sup.4 __________________________________________________________________________ J/m.sup.2 Low Carbon Steel (U.S. spec. A1S1B1113) Low strength, free 7.80 2.06 383 33 68 8.7 cutting steel [c = 0.1%; Si = Tr; Mn = 1.0%; s = 0.42%; P = 0.4%] Plain Carbon Steel (U.S. spec. A1S1C1038) 7.82 2.06 (a) Normalized at 860.degree. C. 7.82 2.06 595 32 38 4.9 (b) Oil quenched from 860.degree. C.; Tempered at 205.degree. C. 7.82 2.06 772 19 5 0.6 (c) Oil quenched from 860.degree. C.; Tempered at 640.degree. C. 7.82 2.06 650 40 122 15.6 640.degree. C. Medium strength for general engineering applications [c = 0.4%; Si = 0.20%; Mn = 0.80%; S = 0.06%] Alloy Steel (U.S. A1S14340) 7.86 2.08 Tool steel, high strength applications (a) Oil quenched from 830.degree. C. 7.86 2.08 1717 10 8.14 1.04 (b) Oil quenched from 830.degree. C,; Tempered at 200.degree. C. 7.86 2.08 1778 14 37 4.71 200.degree. C. (c) Oil quenched from 830.degree. C,; Tempered at 600.degree. C. 7.86 2.08 1134 18 64 8.14 [c = 0.4%; Si = 0.2%; Mn = 0.5%; Ni = 1.5%; Cr = 1.2%; Mo = 0.3%] Aluminum (ASTM B209/- 1100) 2.7 0.68 80 30 37 13.7 Annealed; low strength corrosion resistant applications Aluminum Alloy (ASTM B209/3003) 2.73 0.69 110 23 54 19.8 1.2% Mn Annealed; castings, medium strength applications Aluminum Alloy (ASTM B247/2018) 2.75 0.69 390 10 13.6 4.9 [4% Cu; 0.8% Mg; 2% Ni] Solution treated and aged; High strength alloy for structural parts Aluminum Alloy (ASTM B85SG100A) 2.65 0.71 320 3.5 2.03 0.77 10% Si; 0.5% Mg Casting alloy (engine parts) medium strength Wood (Average) 0.5 0.1* 130* 2* 1** 2** *Properties in the grain direction **Crack propagation across the grain Glass Reinforced Plastics Undirectional 1.8 0.37 500 1.6 1 0.6 parallel fibers (50% volume fraction) Two directional cloth 1.8 0.20 180 1.3 1 0.6 (50% volume fraction) __________________________________________________________________________
Very little of the failure energy is believed to be absorbed by either the fibers or the matrix, individually. Such observations perhaps explain why some prior art efforts to improve the work-to-fracture properties of FRP composites have focused on enhancement of the fiber pull-out mechanism of failure energy absorption.
Representative of prior art fiber pull-out enhancement techniques is the use of a debonding or wetting agent on polyaramid fibers prior to impregnation by a polyester or epoxy resin matrix. Another technique is the use of matrix resins which shrink from the composite fibers during cure.
British Pat. No. 1,331,431 issued Sept. 26, 1973, to J. G. Morley, teaches the use of convoluted or helically wound fiber drawn into a matrix bonded cylindrical bore to increase the extent of fiber/matrix frictional work.
Another British Patent to J. G. Morley, No. 1,333,711 issued Oct. 17, 1973, discloses the FRP composite fabrication method of weakly bonding a strong fiber to a sheath which is, respectively, strongly bonded to the matrix.
M. D. Campbell, in U.S. Pat. No. 4,265,981, cites the prior Morley techniques of fiber pull-out enhancement relative to Campbell's improvement of helically wrapping a relatively weak material about stronger fiber reinforcing elements prior to matrix bonding the wrapped fibers in a composite.
Proceeding from a different theory of failure energy absorption, J. E. Gordon and G. Jeronimidis published "Composites With High Work Of Fracture," Phil. Trans. R. Soc. Lond. A294, 545-550 (1980). Gordon and Jeronimidis analyzed the failure mechanics of natural wood to postulate the synergistic strength and toughness of that material as derived from a stress induced tensile buckling of helically wound hollow cellulose fiber. Under tensile stress, the helically wound cellulose cell walls initially collapse inwards, thereby severing the lignin matrix bond between cells. This facilitates a significant axial extension as the helical fibrils straighten and shear leading to the ultimate failure of the cell.
From these observations of natural wood failure, Gordon and Jeronimidis suggested an FRP composite of helically wound man-made fibrous elements such as glass or carbon into hollow tubes, the fibers within the tube walls and the tubes themselves being bonded together by a polymeric matrix; the tube cores remaining empty.
Gordon and Jeronimidis additionally discovered the optimum helix angle of 15.degree. at which their hollow fibrous tubes should be wound. Within limits, larger angles may provide higher values of work-to-fracture but lead to unacceptable losses in strength and stiffness. On the other hand, smaller angles significantly reduce work-to-fracture for marginal improvements in strength and stiffness. Using such optimally wound tubes, composites yielding specific work-to-fracture values of 40.times.10.sup.4 J/m.sup.2 were obtained.
With no apparent recognition of the fundamental mechanics involved as proposed by Gordon and Jeronimidis, L. E. Trenner disclosed in U.S. Pat. No. 3,146,155 a method of fabricating flat, structural panels comprising a core layer of helically wound, open hollow coils formed of resin bonded glass fiber. The core layer was laminated between two facing sheets of resin bonded fiberglass matting.
Although Gordon and Jeronimdis have proven a theoretical basis for fabricating remarkably high strength, stiffness and work-of-fracture FRP composites, the necessity of helically winding massive quantities of resin saturated fibers into hollow tube elements for assembling such composites is economically unattractive at the present state-of-the-art. It is therefore, an object of the present invention to disclose and teach a method of synthesizing helically wound hollow tubes that is compatible with the present state-of-the-art mass production techniques.
Another object of the present invention is to teach a method of fabricating extremely strong, light and tough composite panels from both fibrous and homogeneous web or sheet base materials.
Another object of the present invention is to provide panel articles of light weight, high strength and toughness, and low manufacturing cost.